Lyme disease is the most commonly reported arthropod-borne disease in the United States and the number of reported cases has doubled to over 20,000 per year during the past decade. Lyme disease is caused by the bite of a tick infected with the spirochete, Borrelia burgdorferi, the etiologic agent of the disease. During previous funding periods, we have determined the genetic diversity among clinical isolates of B. burgdorferi and found a significantly different distribution of genotypes in isolates obtained from skin compared with blood. Furthermore, a particular genotype that is readily cultivated from skin specimens is rarely cultured from blood. These findings demonstrate that different genotypes of B. burgdorferi possess varying potential for dissemination in an infected host. In preliminary studies, we have observed that exposure of human peripheral blood mononuclear cells (PBMCs) to B. burgdorferi results in robust induction of a type I interferon (IFN) response that is mediated through toll-like receptor (TLR)-7 and TLR-9. Furthermore, B. burgdorferi strain variants lacking linear plasmids, in particular lp36, cannot induce this response. We hypothesize that one or more factors encoded by lp36 are specifically recognized by phagocytic cells, leading to uptake and subsequent stimulation of type I IFN production via TLR7/9 signaling from within the phagolysosome and that the ability of a B. burgdorferi strain to induce a type I IFN response may play an important role in spirochetal dissemination. In this revision application we will test this hypothesis by a combination of genetic, cell biology and animal studies. Specifically, the lp36-encoded factor responsible for IFN-a production will be identified, the relative uptake of diverse B. burgdorferi isolates by phagocytic cells will be determined and the correlation between early induction of a type I IFN response and spirochetal dissemination will be explored in a murine Lyme disease model. This revision application is submitted in response to NOTOD- 09-058: "NIH Announces the Availability of Recovery Act Funds for Competitive Revision Applications". The proposed studies clearly build and expand on the aims of the parent grant, will accelerate the tempo of scientific research and allow for job retention. They will shed light on the basis for symptom development in infected patients and on the factors contributing to dissemination of B. burgdorferi in humans - a central goal of the parent grant. PUBLIC HEALTH RELEVANCE: Lyme disease, the most commonly reported arthropod-borne disease in the United States is caused by the bite of a tick infected with the spirochete, Borrelia burgdorferi, the etiologic agent of the disease. We have observed a novel and previously undescribed TLR7/TLR9-mediated signaling pathway for B. burgdorferi, which results in the production of high levels of type I IFNs by human PBMCs and hypothesize that this response plays a role in spirochetal dissemination. The proposed studies will shed light on the basis for symptom development in infected patients and on the factors contributing to dissemination of B. burgdorferi in humans.